1. Field of the Invention
The present invention relates to a circuit, and more particularly an integrated circuit, that controls both a buck circuit and a full-bridge ballast circuit in an HID ballast.
2. Prior Art
A typical prior art solution for a High-Intensity Discharge (HID) ballast (FIG. 1) includes a boost PFC stage 10, a buck stage 11 and a full-bridge stage 12. The boost PFC stage 10 performs power factor correction (PFC) and regulates the DC bus output voltage to a typical value of 400VDC. The buck stage 11 controls the amount of current being delivered to the lamp 13 such that the lamp runs at its nominal power level. The full-bridge stage 12 drives the lamp 13 at a low frequency (200 Hz typical) and provides the AC voltage waveform across the lamp 13.
The boost stage 10 is typically controlled with a standard PFC control IC 14, of which several are marketed by various IC manufacturers. The buck stage 11 is controlled by a pulse-width modulation (PWM) IC 15 for controlling the on-time and therefore the amount of current being delivered to the lamp load 13. A standard PWM control IC 15 or discrete control circuitry is traditionally used for this and generates a gate drive signal which is then level-shifted to the buck MOSFET 16 using a standard high-voltage gate-driver IC 17 such as the International Rectifier IR2117. The full-bridge stage 12 is typically controlled using two International Rectifier IR2153 self-oscillating gate driver ICs 18. Discrete control circuitry 19 is typically used to control the full-bridge stage 12 and the PWM IC15 and more specifically performs the following functions:                1) Lamp ignition (on/off control of the ignitor circuit 20)        2) Senses lamp voltage and current        3) Sets the on-time reference for the buck controller 15        4) Detects various lamp fault conditions        5) Provides ignition timing        6) Counts the number of fault events        7) Resets the ballast or turns the ballast off when faults occur or when the fault counter has timed out.        
This solution typically requires six or more control ICs resulting in a high component count, large area of PCB board space, high manufacturing costs, and high overall ballast cost. A more elegant solution is desired for integrating as many functions as possible into a single IC to reduce component count, reduce PCB board space, reduce manufacturing costs, reduce overall ballast cost, and increase reliability.